1. Technical Field
Exemplary aspects of the present invention relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing an image on a recording medium and an image forming apparatus incorporating the fixing device.
2. Description of the Background
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having two or more of copying, printing, scanning, facsimile, plotter, and other functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of a photoconductor; an optical writer emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a development device supplies toner to the electrostatic latent image formed on the photoconductor to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the photoconductor onto a recording medium or is indirectly transferred from the photoconductor onto a recording medium via an intermediate transfer belt; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
The fixing device may employ an induction heater to heat the recording medium quickly. For example, the induction heater heats a fixing rotary body, such as a fixing roller and a fixing belt, pressingly contacted by a pressure roller to form a fixing nip therebetween. As the recording medium bearing the toner image is conveyed through the fixing nip, the fixing rotary body and the pressure roller apply heat and pressure to the recording medium, thus melting and fixing the toner image on the recording medium. Since the fixing rotary body incorporates a heat generation layer that generates heat by a magnetic flux generated by an exciting coil of the induction heater, the fixing rotary body is heated to a desired fixing temperature to fix the toner image on the recording medium quickly.
However, the heat generation layer is thin and therefore may cause temperature variation of the fixing rotary body in an axial direction thereof. For example, after a plurality of small recording media is conveyed over the fixing rotary body continuously, both lateral ends of the fixing rotary body in the axial direction thereof may overheat because the small recording media are not conveyed over both lateral ends of the fixing rotary body and therefore do not draw heat therefrom. Accordingly, the temperature of the fixing rotary body varies in the axial direction thereof. Consequently, as a large recording medium is conveyed over the fixing rotary body immediately after conveyance of the small recording media, temperature variation of the fixing rotary body may vary gloss of a toner image on the large recording medium.
In order to eliminate temperature variation of the fixing rotary body, two solutions are proposed.
As a first solution, a self temperature control to offset a magnetic flux with a repulsive magnetic flux may be used. For example, a magnetic shunt alloy may be interposed between the heat generation layer and a metal plate serving as a degausser. When the temperature of the magnetic shunt alloy reaches a Curie temperature, a magnetic flux from the exciting coil penetrates the metal plate, allowing the metal plate to generate a repulsive magnetic flux that offsets the magnetic flux from the exciting coil.
As a second solution to eliminate temperature variation of the fixing rotary body, a magnetic flux shield may be interposed between the exciting coil and the fixing rotary body incorporating a heat generator. The magnetic flux shield is movable in a circumferential direction of the fixing rotary body and has a shape that adjusts an amount of magnetic fluxes directed to the fixing rotary body from the exciting coil.
However, the magnetic flux shield interposed between the exciting coil and the heat generator of the fixing rotary body may occupy a substantial space that increases an interval between the exciting coil and the heat generator, degrading heat generation efficiency of the heat generator. The increased interval between the exciting coil and the heat generator may increase an interval between the exciting coil and the magnetic flux shield, degrading degaussing efficiency of the magnetic flux shield. The degraded heat generation efficiency may make it longer for the fixing rotary body to be warmed up to a predetermined fixing temperature.
Further, the magnetic flux shield movable in the circumferential direction of the fixing rotary body may complicate the configuration of the fixing device, upsizing the fixing device.
Additionally, in order to suppress overheating of both lateral ends of the fixing rotary body in the axial direction thereof where the small recording media are not conveyed, it may take a substantial time to detect the temperature of both lateral ends of the fixing rotary body in the axial direction thereof and move the magnetic flux shield in the circumferential direction of the fixing rotary body.